(a) Field of the Invention
The present invention relates to an amplifier with power supply switching arranged so that power supply voltages which are selectively supplied to an amplifying device are switched in accordance with the voltage level of a signal which is amplified in the amplifier, to attain both improved power efficiency and reduction of signal distortion.
(b) Description of the Prior Art
As power amplifier for use in audio devices and designed to improve power efficiency, there has been known one which is arranged so that power supply voltages supplied to the power amplifier are switched between high level and low level in accordance with the output voltage derived from this power amplifier.
FIG. 1 shows an example of such conventional power amplifier. In the Figure, reference numeral 1 represents an amplifying circuit whose output voltage is supplied to an absolute value circuit 2 and to a loudspeaker 3. Arrangement is provided so that high voltage power supplies 4a, 4b or low voltage power supplies 5a, 5b are brought into connection to said amplifying circuit 1 through respective changeover circuits 6a or 6b.
The switching actions of these changeover circuits 6a and 6b are controlled by a changeover controlling circuit 9 based on an output delivered from a comparator 8 which compares the output voltage of the absolute value circuit 2 with a reference voltage V.sub.R of a reference power supply 7, one end of which is grounded. More specifically, the comparator 8 functions so that, when the absolute value of the output voltage of the amplifying circuit 1 is lower than the reference voltage V.sub.R of said reference power supply 7, the comparator 8 delivers the result of the comparison to the changeover control circuit 9. Whereby, the changeover control circuit 9 controls the respective changeover circuits 6a and 6b to cause the low voltage power supplies 5a and 5b to supply their output voltages to the amplifying circuit 1. Conversely, in case the absolute value of the output voltage of the amplifying circuit 1 is higher than the reference voltage V.sub.R, the high voltage power supplies 4a and 4b are then brought into connection to the amplifying circuit 1 via the changeover circuits 6a and 6b, respectively.
In the abovesaid known power amplifier shown in FIG. 1, the reference power supply 7 is arranged so as to use the ground level as its reference voltage. Therefore, this amplifier has the following inconveniences. That is, with regard to the voltages of the low voltage power supplies 5a and 5b, there would arise variation of current supplied to the amplifying circuit 1 from these low voltage power supplies 5a and 5b in accordance with the signal level of the amplifying circuit 1, and such variation of current also serves to the development of voltage fluctuation (voltage drop) of these power supplies. Let us here consider, for example, the instance wherein the voltage level of the output signal undergoes a gradual increment. Such increment of voltage level of the output signal brings about an increase in the current flowing into the amplifying circuit 1 from the low voltage power supplies 5a and 5b, and such increase in the current, in turn, causes a gradual drop of voltages of these power supplies 5a and 5b. Accordingly, in the vicinity of a voltage level at which the output signal voltage is about to reach the reference voltage level V.sub.R of said reference power supply 7, the voltage values of the low voltage power supplies 5a and 5b will drop to such extent as will cause saturation of the amplifying device of the amplifying circuit 1, causing distortion of the signal to develop. Such saturation of the amplifying device could occur also due to a voltage drop during voltage fluctuation of the commercial ac power supply which constitutes the source of power supply to the amplifier.
As a solution to such problems encountered in the past, there can be considered a method of preventing the saturation of the amplifying device by setting, at a sufficiently low level, the reference voltage of the reference power supply 7, so as to advance the timing of switching of connection from the low voltage power supplies over to the high voltage power supplies in case the voltage level of the output signal is rising. Alternatively, the power supply voltage per se are set at sufficiently high levels to thereby prevent the saturation of the amplifying device. These efforts, on the one hand, are effective for the prevention of development of signal distortion. However, on the other hand, they result in an increase in power loss in said amplifying circuit, causing degradation of the power efficiency of the power amplifier.